In a spread spectrum communication system, downlink transmissions from a base station to a mobile station include a pilot channel and a plurality of traffic channels. The pilot channel is decoded by all users. Each traffic channel is intended for decoding by a single user. Therefore, each traffic channel is encoded using a code known by both the base station and mobile station. The pilot channel is encoded using a code known by the base station and all mobile stations. Encoding the pilot and traffic channels spreads the spectrum of transmissions in the system.
One example of a spread spectrum communication system is a cellular radiotelephone system according to Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) Interim Standard IS-95, "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System"("IS-95"). Individual users in the system use the same frequency but are distinguishable from each other through the use of individual spreading codes. Other spread spectrum systems include radiotelephone systems operating at 1900 MHz, commonly referred to as DCS1900. Other radio and radiotelephone systems use spread spectrum techniques as well.
IS-95 is an example of a direct sequence code division multiple access (DS-CDMA) communication system. In a DS-CDMA system, transmissions are spread by a pseudorandom noise (PN) code. Data is spread by chips, where the chip is the spread spectrum minimal-duration keying element.
Mobile stations for use in spread spectrum communication systems have employed RAKE receivers. A RAKE receiver is a form of matched filter receiver which includes two or more receiver fingers which independently receive radio frequency (RF) signals. Each finger estimates channel gain and phase and demodulates the RF signals to produce traffic symbols. The traffic symbols of the receiver fingers are combined in a symbol combiner to produce a received signal.
A RAKE receiver is used in spread spectrum communication systems to combine multipath rays and thereby exploit channel diversity. Multipath rays include line of sight rays received directly from the transmitter and rays reflected from objects and terrain. The multipath rays received at the receiver are separated in time. The time separation or time difference is typically on the order of several chip times. By combining the separate RAKE finger outputs, the RAKE receiver achieves path diversity.
Generally, the RAKE receiver fingers are assigned to the strongest set of multipath rays. That is, the receiver locates local maxima of the received signal. A first finger is assigned to receive the strongest signal, a second finger is assigned to receive the next strongest signal, and so on. As received signal strength changes, due to fading and other causes, the finger assignments are changed. After finger assignment, the time locations of the maxima change slowly, and these locations are tracked by time tracking circuits in each assigned finger.
One limitation on the performance of a DS-CDMA receiver is multiple-access interference or noise at the receiver. Generally, there are two sources of multiple-access interference on the forward link, from base station to the subscriber unit. The first source is multipath originating from the same base station or the same sector of the same base station as the received signal of interest. The multiple traffic signals transmitted from the base station are orthogonal at the base station's transmitter, because the covering Walsh codes are orthogonal. In the RAKE receiver, interference from orthogonal received traffic signals is completely suppressed. However, multipath in the channel between the base station and the receiver destroys the orthogonality of the Walsh codes by introducing time delay. As a result, some multiple-access interference is introduced.
The second source of multiple-access interference is interference from other sectors, both those sectors in soft-handoff with the subscriber unit and those not in soft-handoff with the subscriber unit. The signals transmitted from neighboring sectors are not orthogonal, regardless of the channel, so some multiple-access interference is introduced at the receiver. Under these conditions, the RAKE receiver performance is limited by multiple access interference.
Accordingly, there is a need in the art for an improved interference suppression technique for DS-CDMA systems.